FIELD OF THE INVENTION
This invention relates to hydraulically operated drills for drilling rock material and the like, and more particularly to a system for automatically controlling the feed and rotational speed of the drill as a function of the drillability of the material being drilled.
Drills employed in the drilling of rock material and other earth formations encounter rock or earth strata of varying drillability. Where the drill is provided with a constant thrust force moving the drill axially into the material being drilled or insuring drill feed at a constant speed, there inherently results a short life for the drill bits when the drill bits encounter relatively difficult drilling material.
It has been proposed to optimize the drilling production rate or feed speed relative to bit wear for a drill which encounters a wide variation of materials with different drilling characteristics. Attempts have been made, therefore, to automatically control the feed rate into a material and the rotational speed of the drill as a function of the drillability of material being drilled. Patents representative of such attempts are U.S. Pat. No. 3,385,376 to Hobhouse issuing May 28, 1976, and U.S. Pat. No. 3,593,807 to Klima issuing July 20, 1971.
In the Hobhouse patent, the system is characterized by a hydraulic motor for rotating the drill, a hydraulic feed motor for controlling the feed of the drill relative to the material being drilled, and a hydraulic motor for driving a drill fluid supply pump supplying fluid or lubricant to the drill. The motors are interconnected so that in operation the feed of pressure fluid to operate the feed motor and the motor driving the drilling fluid supply pump are under control of sequence valves, which sequence valves are operated by the pressure existing in the feed of the pressure fluid to the drill motor, to thereby control the feed of the drill and supply drilling fluid to the drill dependent upon the resistance to rotation of the drill.
In the patent to Klima, an electrical motor is employed for rotating the drill bit, while a hydraulic feed motor feeds the rotating drill bit relative to the rock formation or other formation being drilled. The invention is characterized by a control system for the drilling equipment which controls the downward force exerted upon the drill bit in accordance with a signal derived from the torque required to rotate the drill at a constant speed and a signal proportional to a desired drill rate. Further, a hydraulic control valve is provided responsive to the signal from the comparator to increase or decrease the flow of hydraulic fluid to the hydraulic drill feed motor and to thereby increase or decrease the downward force acting on the drill bit in accordance with the sense of the resultant signal determined in the comparator.
Both systems are, therefore, characterized by a control in which the thrust of the feed motor is increased and the feed rate decreased as the drill enters material more difficult to drill, such that the feed rate and the rotational speed of the drill are in accordance with the drillability of material being drilled.
The present invention is, therefore, directed to an improvement in a control system for controlling the feed rate and rotational speed of a drill in accordance with the drillability of material being drilled, to insure operation of the drill rotation motor at a minimum rotational speed and to insure a proportional reduction in speed of the rotation motor in response to a decrease in speed of the feed motor in an automatic manner upon the drill bit contacting rock material or other work piece whose drillability is of increasing difficulty.